The goals of this project are to elucidate the mechanisms of action of tumor viruses and to search for evidence of tumor viruses in humans with the eventual aim of developing rational approaches to prevention of human cancer. Topics of present interest include: 1) the biology of endogenous retroviruses, 2) the molecular biology of retrovirus replication and transformation, and 3) the application of knowledge gained from these studies to the search for tumor viruses of humans.